Method for timing detection

ABSTRACT

A wireless communication system comprises a sampling module that samples a first portion and a second portion of a channel using a sampling time. A correlator module selectively correlates the first portion and the second portion, generates correlation samples, and calculates an offset based on the correlation samples. A timing module selectively adjusts the sampling time based on the offset.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/685,152, filed on May 27, 2005. The disclosure of the aboveapplication is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to communication systems, and moreparticularly to systems and methods for demodulating data in wirelesscommunication systems.

BACKGROUND OF THE INVENTION

Personal Handy-phone System (PHS) is a mobile telephone system thatoperates in the 1.88-1.93 GHz frequency band. PHS is a cordlesstelephone system with capability to handover signals from one cell toanother. PHS cells are smaller than cells of cellular phone systems thatuse Global System for Mobile communication (GSM).

Typically, PHS has a transmission power of 500 mW and a range of 10-100meters. PHS provides service with minimal congestion in areas of heavycall-traffic such as business districts, downtown, etc. This isaccomplished by installing cell stations at a radial distance of every100-200 meters. Thus, PHS is particularly suitable for use in urbanareas.

PHS-based phones can be used in homes, offices, and outdoors. PHS offersa cost-effective alternative to conventional phone systems that useground lines. Additionally, PHS-based phones can interface withconventional phone systems. Thus, where ground lines of conventionalphone systems cannot reach a physical location of a subscriber, thesubscriber can use PHS to establish communication with the conventionalphone system and reach other subscribers served by the conventionalphone system.

PHS uses Time division multiple access (TDMA) as radio interface andadaptive differential pulse code modulation (ADPCM) as voicecoder-decoder (codec). A codec includes an analog-to-digital converter(ADC) and a digital-to-analog converter (DAC) that translate signalsbetween analog and digital formats.

TDMA is a digital signal transmission scheme that allows multiple usersto access a single radio-frequency (RF) channel. Interference betweenchannels is avoided by allocating unique time slots to each user withineach channel. For example, a PHS frame comprises four channels: onecontrol channel and three traffic channels. Each channel is divided intotwo time slots. The control channel assigns each caller one time slotfor uplink or transmission and one time slot for downlink or reception.

Unlike PCM codecs that quantize speech signals directly, ADPCM codecsquantize a difference between a speech signal and a prediction made ofthe speech signal. If the prediction is accurate, the difference betweenactual and predicted speech may have a variance that is lower than thevariance in actual speech. Additionally, the difference may beaccurately quantized with fewer bits than the number of bits that wouldbe needed to quantize the actual speech. While decoding, a quantizeddifference signal is added to a predicted signal to reconstruct anoriginal speech signal. The performance of the codec is aided by usingadaptive prediction and quantization so that a predictor and adifference quantizer adapt to changing characteristics of speech beingcoded.

Referring now to FIG. 1, an exemplary PHS phone 10 comprises an antenna12, a signal processing module 16 comprising a transmit module 18 and areceive module 20, memory 22, a power supply 24, and an I/O module 26.The I/O module 26 may comprise various user-interfaces such as amicrophone 26-1, a speaker 26-2, a display 26-3, a keypad 26-4, a camera26-5, etc.

The transmit module 18 converts user input from the microphone 26-1, thecamera 26-5, etc., into PHS-compatible signals. The receive module 20converts data received from the antenna 12 into a user-recognizableformat and outputs the same via speaker 26-2, camera 26-5, etc. Thesignal processing module 16 uses memory 22 to process data transmittedto and received from the antenna 12. The power supply 24 provides powerto the phone 10.

Digital data is typically represented by zeros and ones, which arecalled bits. Data is generally transmitted by modulating amplitude,frequency, or phase of a carrier signal with a basebandinformation-bearing signal. Quadrature phase shift keying (QPSK) is aform of phase modulation generally used in communication systems. InQPSK, information bits are grouped in pairs called dibits. Thus, QPSKuses four symbols that represent dibit values 00, 01, 10, and 11. QPSKmaps the four symbols to four fixed phase angles. For example, symbol 00may be mapped to (+3π/4). On the other hand, π/4-DQPSK uses differentialencoding wherein mapping between symbols and phase angle varies.Additionally, π/4-DQPSK maps each of the four symbols to a real and animaginary phase angle resulting in an eight-point constellation.

Referring now to FIGS. 2A-2B, the transmit module 18 comprises an ADPCMmodule 50, a framer module 52, a serial-to-parallel converter module 54,a DQPSK mapper module 56, a square-root raised cosine (SRRC) filtermodule 58, and an upsample module 60. The receive module 20 comprises adownsample module 70, an automatic gain control (AGC) module 72, ademodulator 75 comprising a carrier acquisition module 74 and anequalization module 76, a de-mapper and parallel-to-serial convertermodule 78, a de-framer module 80, and an ADPCM module 82.

When transmitting data from the phone 10 on a channel, the ADPCM module50 converts audio and/or video signal into bits of digital data. Theframer module 52 partitions the digital data into frames. Theserial-to-parallel converter module 54 converts the bits in the framesinto symbols. The DQPSK mapper module 56, which may utilize a modulationscheme such as π/4-DQPSK modulation, maps four real and four imaginaryvalues of four symbols in each frame to a total of eight phase anglesand generates a complex baseband signal.

The SRRC filter module 58, which is essentially a Nyquist pulse-shapingfilter, limits the bandwidth of the signal. Additionally, the SRRCfilter module 58 removes mixer products from the complex basebandsignal. The upsample module 60 comprises a quadrature carrier oscillatorthat is used to convert the phase-modulated baseband signal into aphase-modulated carrier signal. The upsample module 60 transmits thephase-modulated carrier signal on the channel at a sampling frequencythat is greater than twice the Nyquist frequency.

When the phone 10 receives a signal from the antenna 12, the downsamplemodule 70 downsamples the signal using an asynchronous oscillator. Thedownsample module 70 down-converts the signal from the phase-modulatedcarrier signal to the phase modulated baseband signal. The AGC module 72maintains the gain of the signal relatively constant despite variationin input signal strength due to transmission losses, noise,interference, etc.

The carrier acquisition module 74 demodulates the signal, retrievescarrier phase information, and decodes symbol values from the signal.The equalization module 76 corrects any distortion present in thesignal. The de-mapper and parallel-to-serial converter module 78 de-mapsand converts the demodulated signal into a serial bit-stream. Thede-framer module 80 de-partitions the frames into digital data bits. TheADPCM module 82 converts the digital data bits into audio and/or videodata and outputs the data to the speaker 26-2 and/or the display 26-3 ofthe phone 10.

SUMMARY OF THE INVENTION

A wireless communication system comprises a sampling module that samplesa first portion and a second portion of a channel using a sampling time.A correlator module selectively correlates the first portion and thesecond portion, generates correlation samples, and calculates an offsetbased on the correlation samples. A timing module selectively adjuststhe sampling time based on the offset.

In other features, the first portion is a preamble (PR) and the secondportion is a unique word (UW). The correlator module selectivelycorrelates the first portion and the second portion when the channel isa traffic channel. The timing module adjusts the sampling time based onthe offset when the channel is a traffic channel. The timing moduleadjusts the sampling time based on the first portion when the channel isa control channel.

In other features, the correlation module generates a correlationwaveform using the correlation samples, determines a peak of thecorrelation waveform, and calculates the offset based on a distance ofthe peak from one of the correlation samples.

In other features, the correlation module generates a correlationwaveform using the correlation samples and determines a peak of thecorrelation waveform using parabolic curve fitting, and calculates theoffset based on a distance of the peak from one of the correlationsamples.

In other features, the correlator module correlates the first portionand the second portion for each time slot of the channel. The correlatormodule correlates using training symbols in the channel. The channelcomprises a plurality of time slots and wherein the timing modulecalculates the offset for each of the time slots. The correlator moduleestimates a location of the second portion in a traffic channel based onthe sampling time of a control channel and based on a burst detected inthe control channel.

In other features, a burst detector module communicates with thesampling module, detects a burst in a control channel, and generates aburst-detect signal when a moving average of one of N phases is lessthan a predetermined threshold. The one of N phases is an array of Nthsample of every symbol in a signal sampled at a sampling rate of Nsamples per symbol, and N is an integer greater than 1. The burst-detectsignal activates the timing module. A carrier offset estimator modulecommunicates with the burst detector module and generates a carrieroffset for a control channel when activated by the burst-detect signal.

In other features, a carrier recovery module communicates with thesampling module and uses a phase-locked loop (PLL) to recover a carriersignal from a sample output by the sampling module. The PLL isinitialized with the carrier offset and wherein the carrier offset isupdate when the PLL locks for each time slot in the channel. Adifferential decoder module communicates with the carrier recoverymodule and decodes a symbol from the sample. The sampling module usesone of a cubic interpolator and a parabolic interpolator. An outputsample rate of the sampling module is equal to a symbol rate and whereinan input sample rate of the sampling module is an integer multiple ofthe symbol rate. A personal handy-phone system (PHS) receiver comprisesthe wireless communication system.

A wireless communication system comprises sampling means for sampling afirst portion and a second portion of a channel using a sampling time.Correlator means selectively correlates the first portion and the secondportion, generates correlation samples, and calculates an offset basedon the correlation samples. Timing means selectively adjusts thesampling time based on the offset.

In other features, the first portion is a preamble (PR) and the secondportion is a unique word (UW). The correlator means selectivelycorrelates the first portion and the second portion when the channel isa traffic channel. The timing means adjusts the sampling time based onthe offset when the channel is a traffic channel. The timing meansadjusts the sampling time based on the first portion when the channel isa control channel. The correlation means generates a correlationwaveform using the correlation samples, determines a peak of thecorrelation waveform, and calculates the offset based on a distance ofthe peak from one of the correlation samples. The correlation meansgenerates a correlation waveform using the correlation samples anddetermines a peak of the correlation waveform using parabolic curvefitting, and calculates the offset based on a distance of the peak fromone of the correlation samples.

In other features, the correlator means correlates the first portion andthe second portion for each time slot of the channel. The correlatormeans correlates using training symbols in the channel. The channelcomprises a plurality of time slots and wherein the timing meanscalculates the offset for each of the time slots.

In yet other features, the correlator means estimates a location of thesecond portion in a traffic channel based on the sampling time of acontrol channel and based on a burst detected in the control channel.Burst detector means communicates with the sampling means, detects aburst in a control channel, and generates a burst-detect signal when amoving average of one of N phases is less than a predeterminedthreshold, wherein the one of N phases is an array of Nth sample ofevery symbol in a signal sampled at a sampling rate of N samples persymbol, and N is an integer greater than 1. The burst-detect signalactivates the timing means. Carrier offset estimator means communicateswith the burst detector means and generates a carrier offset for acontrol channel when activated by the burst-detect signal. Carrierrecovery means communicates with the sampling means and usesphase-locked loop (PLL) means for recovering a carrier signal from asample output by the sampling means. The PLL is initialized with thecarrier offset. The carrier offset is updated when the PLL locks foreach time slot in the channel.

In other features, differential decoder means communicates with thecarrier recovery means and decodes a symbol from the sample. Thesampling means uses one of a cubic interpolator and a parabolicinterpolator. An output sample rate of the sampling means is equal to asymbol rate and wherein an input sample rate of the sampling means is aninteger multiple of the symbol rate. A personal handy-phone system (PHS)receiver comprising the wireless communication system.

A computer program executed by a processor for operating wirelesscommunication system comprises sampling a first portion and a secondportion of a channel using a sampling time; selectively correlating thefirst portion and the second portion; generating correlation samples;calculating an offset based on the correlation samples; selectivelyadjusting the sampling time based on the offset.

In other features, the first portion is a preamble (PR) and the secondportion is a unique word (UW). The computer program selectivelycorrelates the first portion and the second portion when the channel isa traffic channel. The computer program includes adjusting the samplingtime based on the offset when the channel is a traffic channel. Thecomputer program includes adjusting the sampling time based on the firstportion when the channel is a control channel. The computer programincludes generating a correlation waveform using the correlationsamples; determining a peak of the correlation waveform; and calculatingthe offset based on a distance of the peak from one of the correlationsamples.

In other features, the computer program includes generating acorrelation waveform using the correlation samples; determining a peakof the correlation waveform using parabolic curve fitting, andcalculating the offset based on a distance of the peak from one of thecorrelation samples. The computer program includes correlating the firstportion and the second portion for each time slot of the channel. Thecomputer program includes correlating using training symbols in thechannel.

In other features, the channel comprises a plurality of time slots andfurther comprising calculating the offset for each of the time slots.The computer program includes estimating a location of the secondportion in a traffic channel based on the sampling time of a controlchannel and based on a burst detected in the control channel. Thecomputer program includes detecting a burst in a control channel; andgenerating a burst-detect signal when a moving average of one of Nphases is less than a predetermined threshold. The one of N phases is anarray of Nth sample of every symbol in a signal sampled at a samplingrate of N samples per symbol, and N is an integer greater than 1.

In other features, the computer program includes adjusting the samplingtime when the burst-detect signal occurs. The computer program includesgenerating a carrier offset for a control channel when the burst-detectsignal occurs. The computer program includes using a phase-locked loopto recover a carrier signal from a sample output. The computer programincludes initializing the PLL with the carrier offset; and updating thecarrier offset when the PLL locks for each time slot in the channel.

In other features, the computer program includes decoding a symbol fromthe sample. The computer program includes using one of a cubicinterpolator and a parabolic interpolator. The computer program includessetting an output sample rate equal to a symbol rate; and setting aninput sample rate equal to an integer multiple of the symbol rate.

In still other features, the systems and methods described above areimplemented by a computer program executed by one or more processors.The computer program can reside on a computer readable medium such asbut not limited to memory, non-volatile data storage and/or othersuitable tangible storage mediums.

A computer method comprises sampling a first portion and a secondportion of a channel using a sampling time; selectively correlating thefirst portion and the second portion; generating correlation samples;calculating an offset based on the correlation samples; selectivelyadjusting the sampling time based on the offset.

In other features, the first portion is a preamble (PR) and the secondportion is a unique word (UW). The computer method selectivelycorrelates the first portion and the second portion when the channel isa traffic channel. The computer method includes adjusting the samplingtime based on the offset when the channel is a traffic channel. Thecomputer method includes adjusting the sampling time based on the firstportion when the channel is a control channel. The computer methodincludes generating a correlation waveform using the correlationsamples; determining a peak of the correlation waveform; and calculatingthe offset based on a distance of the peak from one of the correlationsamples.

In other features, the computer method includes generating a correlationwaveform using the correlation samples; determining a peak of thecorrelation waveform using parabolic curve fitting, and calculating theoffset based on a distance of the peak from one of the correlationsamples. The computer method includes correlating the first portion andthe second portion for each time slot of the channel. The computermethod includes correlating using training symbols in the channel.

In other features, the channel comprises a plurality of time slots andfurther comprising calculating the offset for each of the time slots.The computer method includes estimating a location of the second portionin a traffic channel based on the sampling time of a control channel andbased on a burst detected in the control channel. The computer methodincludes detecting a burst in a control channel; and generating aburst-detect signal when a moving average of one of N phases is lessthan a predetermined threshold. The one of N phases is an array of Nthsample of every symbol in a signal sampled at a sampling rate of Nsamples per symbol, and N is an integer greater than 1.

In other features, the computer method includes adjusting the samplingtime when the burst-detect signal occurs. The computer method includesgenerating a carrier offset for a control channel when the burst-detectsignal occurs. The computer method includes using a phase-locked loop torecover a carrier signal from a sample output. The computer methodincludes initializing the PLL with the carrier offset; and updating thecarrier offset when the PLL locks for each time slot in the channel.

In other features, the computer method includes decoding a symbol fromthe sample. The computer method includes using one of a cubicinterpolator and a parabolic interpolator. The computer method includessetting an output sample rate equal to a symbol rate; and setting aninput sample rate equal to an integer multiple of the symbol rate.

In still other features, the systems and methods described above areimplemented by a computer method executed by one or more processors. Thecomputer method can reside on a computer readable medium such as but notlimited to memory, non-volatile data storage and/or other suitabletangible storage mediums.

Further areas of applicability of the present invention will becomeapparent from the detailed description provided hereinafter. It shouldbe understood that the detailed description and specific examples, whileindicating the preferred embodiment of the invention, are intended forpurposes of illustration only and are not intended to limit the scope ofthe invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description and the accompanying drawings, wherein:

FIG. 1 is a functional block diagram of an exemplary personalhandy-phone system (PHS) phone according to the prior art;

FIG. 2A is a functional block diagram of an exemplary transmitter usedin a PHS phone of FIG. 1 according to the prior art;

FIG. 2B is a functional block diagram of an exemplary receiver used in aPHS phone of FIG. 1 according to the prior art;

FIG. 3 is a functional block diagram of an exemplary demodulator used ina receiver of a PHS phone according to the present invention;

FIG. 4 is a functional block diagram of an exemplary burst detector usedin the demodulator of FIG. 3 according to the present invention;

FIG. 5 is a graph showing a correlation curve with paraboliccurve-fitting according to the present invention;

FIG. 6 is a functional block diagram of an exemplary carrier recoverymodule having automatic frequency control (AFC) that is used in thedemodulator of FIG. 3 according to the present invention;

FIG. 7 is a pi-chart of exemplary simulation test results showingperformance of the demodulator of FIG. 3 according to the presentinvention; and

FIG. 8 is a flowchart for a method used by the demodulator of FIG. 3 todemodulate symbols according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following description of the preferred embodiment(s) is merelyexemplary in nature and is in no way intended to limit the invention,its application, or uses. For purposes of clarity, the same referencenumbers will be used in the drawings to identify similar elements. Asused herein, the term module, circuit and/or device refers to anApplication Specific Integrated Circuit (ASIC), an electronic circuit, aprocessor (shared, dedicated, or group) and memory that execute one ormore software or firmware programs, a combinational logic circuit,and/or other suitable components that provide the describedfunctionality. As used herein, the phrase at least one of A, B, and Cshould be construed to mean a logical (A or B or C), using anon-exclusive logical or. It should be understood that steps within amethod may be executed in different order without altering theprinciples of the present invention.

The present disclosure is applicable to communications systems. Forexample, the present disclosure is applicable to wireless communicationssystems. The present disclosure is also applicable to time divisionmultiple access (TDMA) systems. In the foregoing description, thepresent disclosure discusses a personal handy-phone system (PHS).However, the present disclosure is not meant to be limited to PHS orTDMA systems.

A receiver in a personal handy-phone system (PHS) phone estimatescarrier phase (and frequency), recovers symbol timing, and estimates amost likely value of a received symbol. The receiver utilizes a symboltiming recovery scheme to estimate timing of symbols in a receivedsignal. The receiver obtains correct values of symbols if the receivedsignal is sampled at proper times.

A timing signal determines a time at which the received signal may besampled to retrieve correct values of symbols. After the receiveridentifies correct symbol timing, the receiver samples the receivedsignal at the correct symbol timing. The receiver retrieves carrier andsymbols from samples generated by sampling the received signal at thecorrect symbol timing and estimates most likely values of the symbols.Thereafter, the receiver converts the estimates into dibits.

Some symbol timing recovery schemes may use a phase difference between acurrent sample and a previous sample to determine symbol timing. If aphase estimate is wrong, however, the symbol values may be wrong sincethe receiver may be effectively using a different symbol mapping than asymbol mapping used by a transmitter while transmitting the symbols.

Some transmitters insert a fixed synchronization pattern into modulationwhen transmitting signals. The receiver searches for the pattern andcorrectly recovers symbols. This scheme, however, consumes bandwidththat can otherwise be used to carry data. Alternatively, the receivercan accurately recover symbols by using additional hardware. Addinghardware, however, may increase system cost and decrease systemmarketability.

In the present disclosure, a PHS receiver samples and recovers symbolsfrom received signals with substantial accuracy by using a demodulationscheme that includes phase correlation, parabolic curve fitting, andinterpolation using correct symbol timing. A signal received by the PHSreceiver is downsampled at a sampling rate equal to three times a symbolrate. Thus, each symbol has three samples. The demodulation schemeidentifies best of the three samples to demodulate. The best sample isdecoded to obtain a correct value for each symbol.

Generally, a PHS signal comprises a series of time division multipleaccess (TDMA) frames. Each TDMA frame may be 5 milliseconds (mS) induration with 2.5 mS for uplink or transmission and 2.5 mS for downlinkor reception. Each TDMA frame may comprise four channels: one controlchannel and three traffic channels. Each channel has two time slots: onetime slot for uplink and one time slot for downlink. Thus, each TDMAframe has a total of eight time slots.

The control channel uses a carrier frequency that is different from thecarrier frequency (or frequencies) used by the traffic channels. Thetraffic channels can use same or different carrier frequencies.

Each TDMA frame comprises a preamble (PR) and a unique word (UW) foreach channel. A UW includes identifying information for the PHS. The PHSuses UW as a security feature to authenticate access by subscribers tothe PHS.

Typically, a TDMA signal is transmitted in bursts. A burst generallycomprises an initial increase in amplitude from zero to a normal valuefollowed by modulated data and a decrease in amplitude to zero. Theburst is detected in the control channel, and bit timing is recoveredfor the control channel by utilizing a periodic pattern in the PR. Theburst comprises a sequence of eight training symbols. The sequence isgenerally inserted in the middle of each time slot and is called amidamble. An equalizer in the PHS receiver uses the sequence to reduceinter-symbol interference.

Additionally, the demodulation scheme uses the training symbols forcorrelation in traffic channels. Based on the burst detection in thecontrol channel and the bit timing of the control channel, anapproximate position of the UW in traffic channels is determined.Substantially accurate bit timing for traffic channels is calculated byperforming a phase correlation of PR and UW for each traffic channel.The phase correlation of PR and UW is performed using training symbols,and correlation samples are generated for each time slot. A correlationcurve is generated using correlation samples for each time slot. Thecorrelation curve is fitted onto a parabola, and a peak of the parabolais calculated.

The peak of the parabola approximately equals a peak of the correlationcurve. The peak of the correlation curve corresponds to the best samplefrom which a symbol may be correctly decoded. An x-coordinate of thepeak of the parabola corresponds to a best time to sample a symbol toget the correct value of the symbol.

Additionally, a distance between the peak of the parabola and anadjacent point on the correlation curve corresponds to a distancebetween the best sample and a sample adjacent to the best sample. Atiming offset is calculated based on the distance between the peak ofthe parabola and the adjacent point on the correlation curve. A samplingtime for sampling the symbols is adjusted by the timing offset. Aninterpolator, which is essentially a sampling module, uses adjustedsampling times to sample symbols at correct times. In other words, thedemodulation scheme uses the peak of the parabola to estimate best timesto sample subsequent symbols. Accordingly, best subsequent samples aredemodulated and correct values of symbols are obtained therefrom.

Additionally, a carrier offset is estimated for the control channel, anda carrier phase is offset by an estimated carrier offset value. Theestimated carrier offset value is calculated based on a first-orderdifferentiation of a downsampled signal. Once carrier phase is offsetand locked using automatic frequency control (AFC), subsequent samplesare demodulated at correct times and are decoded to obtain substantiallyaccurate values of the symbols.

For traffic channels, the AFC is initialized with the estimated carrieroffset calculated for the control channel. A frequency step size of theAFC is decreased until the AFC locks. The estimated carrier offset isupdated for each time slot. Samples are demodulated and decoded toobtain substantially accurate values of the symbols.

Referring now to FIG. 3, a demodulation system 75-1 of a PHS phonereceiver comprises an arctangent module A 100, an arctangent module B100-1, a single differentiator module 102, a double differentiatormodule 104, a burst detector module 106, a timing module 108, a UWcorrelator module 110, a sampling module 112, an offset estimator module114, a carrier recovery module 117, and a differential decoder module120.

The demodulation system 75-1 receives a signal that is downsampled at arate of three samples per symbol (3f_(b)). An array of first samples ofevery symbol is called phase 1. An array of second samples for everysymbol is called phase 2. An array of third samples of every symbol iscalled phase 3. The arctangent module A 100 receives the signal at the3f_(b) rate. The arctangent module A 100 recovers a phase angle of thesignal based on in-phase (I) and quadrature (Q) components of thesignal. The arctangent module A 100 outputs the phase angle informationfor phase 1, phase 2, and phase 3 (collectively phases i) to the singledifferentiator module 102. This may be represented byarctangentOut(i−3), where i represents one of phases i.

The sampling module 112 interpolates the three samples of every symbolbased on a sampling signal generated by the timing module 108. Thesampling module 112 outputs one sample called best sample per symbol.Thus, output of the sampling module 112 is at the symbol rate (f_(b)).

The arctangent module B 100-1 receives the output of the sampling module112. The arctangent module B 100-1 recovers a phase angle of the outputof the sampling module 112 and outputs the phase angle information tothe single differentiator module 102 and the carrier recovery module117. The output of the arctangent module B 100-1 may be represented byarctangentOut(i).

The single differentiator module 102 and the double differentiatormodule 104 perform differentiation for each phase separately. Forexample, singleDiff (i)=arctangentOut (i)−arctangentOut (i−3). An outputof the double differentiator module 104 is input to the burst detectormodule 106.

The burst detector module 106 detects a burst in the control channel.The burst detector module 106 adds two contiguous outputs of the doubledifferentiator module 104 for phase i and calculates an absolute valueof a sum of the two contiguous outputs. Moving averages of absolutevalues for each one of phases i are calculated and compared to a burstthreshold ThB. The burst detector module 106 detects the burst if amoving average of any one of phases i is less than ThB.

The burst detector module 106 generates a burst-detect signal thatenables the timing module 108 and the offset estimator module 114. Thetiming module 108 utilizes a periodic pattern in the preamble (PR) inthe control channel instead of performing a phase correlation to recoverbit timing. The timing module 108 determines bit timing and generates acorrect sampling time for the control channel based on the PR in thecontrol channel.

Burst detection is not performed in traffic channels because the PR intraffic channels is not as long as the PR in the control channel.Additionally, the timing module 108 cannot recover bit timing fortraffic channels based on PR alone since the PR in traffic channels isnot of sufficient length. Therefore, approximate bit timing for trafficchannels is initially estimated based on the burst detection and the bittiming of the control channel.

An approximate position of the UW in traffic channels is determinedbased on the burst detection in the control channel and bit timing ofthe control channel. Thereafter, a phase correlation of PR and UW isperformed for each traffic channel and a timing offset calculated.Substantially correct bit timing and sampling time for traffic channelsare obtained by adjusting the approximate bit timing and sampling timesof the control channel by the timing offset.

Specifically, the UW correlator module 110 performs the phasecorrelation of PR and UW for each traffic channel and generatescorrelation samples. The UW correlator module 110 generates acorrelation curve using the correlation samples for each time slot ofeach traffic channel in a frame. The UW correlator module 110 performs aparabolic curve-fitting. That is, the correlation curve is fitted onto aparabola and a peak of the parabola is calculated. The peak of theparabola approximately corresponds to a peak of the correlation curve.

The UW correlator module 110 calculates the timing offset based on adistance between the peak of the parabola and an adjacent correlationsample on the correlation curve. The timing module 108 adjusts the bittiming and the sampling times of the traffic channels by the timingoffset.

The sampling module 112 samples symbols in best of the three samples atsampling times generated by the timing module 108. The sampling module112 essentially interpolates three samples comprising one symbol andsamples the interpolated data at the correct sampling time generated bythe timing module 108. The sampling module 112 effectively generates onesample called the best sample from the three samples, which yields acorrect value of a symbol when decoded.

Thus, an output sample rate of the sampling module 112 is equal to thesymbol rate. The symbol rate is determined by the number of symbols usedin modulation and may be expressed as number of symbols per second.Additionally, by using the correct sampling times, the sampling module112 can estimate substantially correct sampling times to samplesubsequent symbols in the frame.

The offset estimator module 114 estimates a carrier offset for thecontrol channel based on an output of the single differentiator module102. The offset estimator module 114 stores the estimated carrier offsetin a register. A carrier recovery module 117 utilizes automaticfrequency control (AFC), which is essentially a phase-locked loop (PLL).The carrier recovery module 117 initializes the AFC with the estimatedcarrier offset.

Thereafter, the carrier recovery module 117 decreases a frequency stepsize of the AFC based on the estimated carrier offset. When the AFC islocked, the carrier recovery module 117 recovers the carrier signal fromthe best sample output by the sampling module 112 and generates ademodulated output comprising a correct value of the symbol. Adifferential decoder module 120 decodes the demodulated output andgenerates digital data represented by the symbol.

For traffic channels, the carrier recovery module 117 initializes theAFC with the estimated carrier offset calculated for the controlchannel. Thereafter, the carrier recovery module 117 decreases thefrequency step size of the AFC based on the estimated carrier offset.Once the AFC is locked, the carrier recovery module 117 updates theestimated carrier offset for each time slot of each traffic channel.Alternatively, the frequency step size may be fixed, AFC may beperformed for an entire input symbol sequence, and the estimated carrieroffset may be updated at the end of each time slot.

When the AFC is locked, the carrier recovery module 117 recovers thecarrier signal from the best sample output by the sampling module 112and generates a demodulated output comprising the correct value of thesymbol. The differential decoder module 120 decodes the demodulatedoutput and generates digital data represented by the symbol.

Referring now to FIG. 4, the burst detector module 106 detects the burstin the control channel. Specifically, an output of the doubledifferentiator module 104 is input to a serial-to-parallel convertermodule 130, which separates phases i. Adder modules 132-1, 132-2, and132-3 (collectively 132) add two contiguous outputs of the doubledifferentiator module 104 for phases i. Absolute function modules 134-1,134-2, 134-3 (collectively 134) calculate absolute values of outputs ofrespective adder modules 132. Moving average modules 136-1, 136-2, 136-3(collectively 136) calculate moving averages of respective absolutevalues.

A comparator module 138 compares moving averages of phases i to apredetermined burst threshold ThB and generates the burst-detect signalif a moving average of one of the phases i is less than ThB. Theburst-detect signal enables the timing module 108 and the offsetestimator module 114.

The phase correlation, parabolic curve fitting, and interpolationperformed by the demodulation system 75-1 can be mathematicallyexplained as follows. For convenience, explanation is limited to timefield. A signal received by the demodulation system 75-1 can beexpressed by the following equation.r(t)=A(t)e ^(j2πΔƒt+φ(t−εT)) +n(t)where A(t) is the amplitude of the signal, Δƒ is the carrier offset,φ(t) is the phase information of the signal, E is the bit timing offset,and n(t) is Gaussian white noise.

For π/4-DQPSK, the phase information φ(t) can be expressed as follows.φ(t)=φ(t−T)+θ(t)

where θ(t) is the phase angle mapped to symbols (a_(k), b_(k)) in thesignal as shown in the following table. (a_(k), b_(k)) θ(k) (0, 0)  π/4(0, 1) 3π/4 (1, 1) −3π/4  (1, 0) −π/4

After single differentiation by the single differentiator module 102,the phase information is given by the following equation.phsSingleDiff(t)=phaseRx(t)−phaseRx(t−T)=2πΔƒT+φ(t)−φ(t−T)After double differentiation by the double differentiator module 104,the phase information is given by the following equation.phsDoubleDiff(t)=phsSingleDiff(t)−phsSingleDiff(t−T)=φ(t)+φ(t−2T)−2φ(t−T)

The burst detector module 106, the timing module 108, and the carrierrecovery module 117 use algorithms that are based on a periodicity ofthe preamble PR. In the burst detector module 106, inputs to thecomparator module 138 are given by following equations.${{sumBurst}\quad 1} = {\sum\limits_{m = 0}^{M - 1}{{abs}\left( {{{phsDoubleDiff}\left( {t - {mT}} \right)} + {{phsDoubleDiff}\left( {t - {mT} - T} \right)}} \right)}}$${{sumBurst}\quad 2} = {\sum\limits_{m = 0}^{M - 1}{{abs}\left( {{{phsDoubleDiff}\left( {t - {T/3} - {mT}} \right)} + {{phsDoubleDiff}\left( {t - {T/3} - {mT} - T} \right)}} \right)}}$${{sumBurst}\quad 3} = {\sum\limits_{m = 0}^{M - 1}{{abs}\left( {{{phsDoubleDiff}\left( {t - {2{T/3}} - {mT}} \right)} + {{phsDoubleDiff}\left( {t - {2{T/3}} - {mT} - T} \right)}} \right)}}$

If one of sumBurst1, sumBurst2, or sumBurst3 is less than apredetermined threshold ThB, the comparator module 138 generates a burstdetect signal that enables the timing module 108 and the offsetestimator module 114.

The timing module 108 performs bit timing recovery for the controlchannel and the traffic channels as follows. For the control channel,phsDoubleDiff(t) is a signal with a period of 2T. A sampling error, ifany, may be expressed as phsDoubleDiff(t−εT). ExpandingphsDoubleDiff(t−εT) using Fourier series, we get $\begin{matrix}{{{phsDoubleDiff}\left( {t - {ɛ\quad T}} \right)} = {a_{0} + {a_{1}{\cos\left( {2\pi\quad{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + {b_{1}{\sin\left( {2\pi\quad{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} +}} \\{{a_{2}{\cos\left( {2{\pi \cdot 2}{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + {b_{2}{\sin\left( {2{\pi \cdot 2}\quad{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + \cdots}\end{matrix}$ wherea₀ = 0, a_(n) = ∫₀^(2T)phsDoubleDiff(t) ⋅ cos (2π ⋅ nf₀t)𝕕t ≠ 0b_(n) = ∫₀^(2T)phsDoubleDiff(t) ⋅ sin (2π ⋅ nf₀t)𝕕t = 0$f_{0} = \frac{1}{2T}$Therefore, phsDoubleDiff(t − ɛ  T) = a₁cos (2π  f₀(t − ɛ  T)) + a₂cos (2π ⋅ 2f₀(t − ɛ  T)) + ⋯

The output of the timing module 108 for the control channel is given bythe following equation.$ɛ = {\frac{1}{\pi}{\arctan\left( \frac{w}{u} \right)}}$ where$\begin{matrix}{u = {\int_{0}^{2T}{{{{phsDoubleDiff}\left( {t - {ɛ\quad T}} \right)} \cdot {\cos\left( {2{\pi \cdot 2}f_{0}t} \right)}}{\mathbb{d}t}}}} \\{= {\int_{0}^{2T}{\left\lbrack {{a_{1}\quad{\cos\left( {2\pi\quad{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + {a_{2}\quad{\cos\left( {2{\pi \cdot 2}{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + \cdots}\quad \right\rbrack \cdot}}} \\{{\cos\left( {2\pi\quad f_{0}} \right)}{\mathbb{d}t}} \\{= {{{a_{1} \cdot 2}T\quad{\cos\left( {2\pi\quad f_{0}ɛ\quad T} \right)}} = {{a_{1} \cdot 2}T\quad{\cos\left( {2\pi\frac{1}{2T}ɛ\quad T} \right)}}}} \\{= {{a_{1} \cdot 2}T\quad{\cos({\pi ɛ})}}}\end{matrix}$ and $\begin{matrix}{w = {\int_{0}^{2T}{{{{phsDoubleDiff}\left( {t - {ɛ\quad T}} \right)} \cdot {\sin\left( {2{\pi \cdot 2}f_{0}t} \right)}}{\mathbb{d}t}}}} \\{= {\int_{0}^{2T}{\left\lbrack {{a_{1}\quad{\cos\left( {2\pi\quad{f_{0}\left( {t - {ɛ\quad T}} \right)}} \right)}} + {a_{2}\quad{\cos\left( {2{\pi \cdot 2}f_{0}\left( {t - {ɛ\quad T}} \right)} \right)}} + \cdots} \right\rbrack \cdot}}} \\{{\sin\left( {2\pi\quad f_{0}T} \right)}{\mathbb{d}t}} \\{= {{{a_{1} \cdot 2}T\quad{\sin\left( {2\pi\quad f_{0}ɛ\quad T} \right)}} = {{a_{1} \cdot 2}T\quad{\sin\left( {2\pi\frac{1}{2T}ɛ\quad T} \right)}}}} \\{= {{a_{1} \cdot 2}T\quad{\sin({\pi ɛ})}}}\end{matrix}$

On the other hand, for traffic channels, the length of the preamble PRis insufficient to perform burst detection and accurate bit timingrecovery. Therefore, the UW correlator module 110 uses burst detectionand bit timing information of the control channel to estimate UWposition in traffic channels and performs phase correlation of PR and UWto determine the best phase. The AFC in the carrier recovery module 117is locked using the best phase.

Initially, frequency offset information is removed from an output of thesingle differentiator module 102 as follows.phsSingleDiff(t)=phaseRx(t)−phaseRx(t−T)=2πΔƒT+φ(t)−φ(t−T)Thereafter, the UW correlator module 110 performs correlation asfollows.Corr(t) = ∫_(t − 12T)^(t)(phsSingleDiff(t) − 2πΔ  fT) * UWmapping(t)𝕕t

Referring now to FIG. 5, a correlation curve 150 is plotted for thecorrelation Corr(t). Since the peak of the correlation curve correspondsto the best phase, the correlation curve is fitted onto a parabola 152for the purpose of finding the peak of the correlation curve. A parabolais expressed by the following equation.y=ax ² +bx+cwhere a, b, and c are the coefficients of the parabola. The peak of theparabola is calculated based on the coefficients by the followingformula. $ɛ = {- \frac{b}{2a}}$

The coefficients of the parabola 152 can be calculated based oncoordinates of three points on the parabola 152: before peak 154, peak156, and after peak 158. If the coordinates of the three points are(x₁,y₁)=(0,y₁), (x₂,y₂)=(1,y₂), (x₂,y₂)=(2,y₂), the coefficients aregiven by the following equations.${a = \frac{y_{1} - {2y_{2}} + y_{3}}{2}},\quad{b = {y_{2} - y_{1} - a}},\quad{c = y_{1}}$After the peak of the parabola 152 is calculated based on thecoefficients, an x-coordinate of the peak is determined. Thex-coordinate of the peak of the parabola 152 corresponds to the besttime to sample a symbol to get the correct value of the symbol.

The peak of the parabola 152 is used to estimate best samples. Thecorrelation module 110 calculates a timing offset based on a distancebetween the peak of the parabola and an adjacent point on thecorrelation curve. The timing module 108 adjusts the bit timing and thesampling times of the traffic channels by the timing offset. Thesampling module 112 interpolates the three samples, samples interpolateddata at the sampling time adjusted by the timing module 108, andgenerates one sample that comprises the correct value of the symbol.

If the signal is noisy, a cubic interpolator may be used instead of aparabolic interpolator. For example, a 4-point cubic interpolator ismathematically expressed by the following equation.${y(n)} = {\sum\limits_{i = I_{1}}^{I_{2}}{C_{i}{x\left( {I_{1} + I_{2} - i} \right)}}}$${{{where}\quad I_{1}} = {- 2}},\quad{I_{2} = 1},\quad{{{and}\quad C_{i}} = {\prod\limits_{{j = I_{1}},{j \neq i}}^{I_{2}}\frac{t - t_{j}}{t_{i} - t_{j}}}}$$C_{- 2} = {{\frac{1}{6}\mu^{3}} - {\frac{1}{6}\mu}}$$C_{- 1} = {{{- \frac{1}{2}}\mu^{3}} + {\frac{1}{2}\mu^{2}} + \mu}$and  now$C_{0} = {{{\frac{1}{2}\mu^{3}} - \mu^{2} - {\frac{1}{2}\mu} + {1C_{- 2}}} = {{{- \frac{1}{6}}\mu^{3}} + {\frac{1}{2}\mu^{2}} - {\frac{1}{3}\mu}}}$

Generally, μ is within [0,1). Therefore, if the peak E of the parabola152 is less than 0, then ε is modulated with osrRx so that ε is within[0,osrRx).

Referring now to FIG. 6, the carrier recovery module 117 utilizes aphase-locked loop (PLL) that performs automatic frequency control (AFC).The carrier recovery module 117 comprises a phase rotator module 117-10that subtracts π/4 from a π/4-DQPSK signal and generates a DQPSK signal.The carrier offset generated for the control channel by the offsetestimator module 114 is input to an accumulator module 117-2. An addermodule 117-1 adds an output of the accumulator module 117-2 to the DQPSKsignal and outputs a sum to a phase-shift calculator module 117-3 and toa detector module 117-4. The phase-shift calculator module 117-3calculates a carrier phase-shift. The detector module 117-4 detectscodes and provides a feedback to the phase-shift calculator module117-3.

The carrier phase-shift is filtered by a carrier filter module 117-5.The carrier filter module 117-5 may utilize a first-order filter. Afiltered carrier phase-shift is input to a digital fixed frequency (DFF)module 117-6. An output of the DFF module 117-6 is input to the detectormodule 117-4 and to a differentiator module 117-7. The differentiatormodule 117-7 differentiates the output of the DFF module 117-6. Adigital filter module 117-8 filters a differentiated signal output bythe differentiator module 117-7 and provides a feedback to the carrierfilter module 117-5. An up-down counter module 117-9 counts add timesand subtract times and provides an output to the accumulator module117-2. When add times and subtract times of the up-down counter module117-9 are nearly equal, the AFC frequency is locked, and the carrierfilter module 117-5 recovers the carrier signal.

The carrier recovery module 117 receives the carrier offset generated bythe offset estimator module 114. The offset estimator module 114calculates the carrier offset for the control channel based on an outputof the single differentiator module 102. The output of the singledifferentiator module 102, phsSingleDiff (t), is a periodic function ofperiod 2T and is expressed by the following equation.phsSingleDiff(t)=phaseRx(t)−phaseRx(t−T)=2πΔƒT+φ(t)−φ(t−T)

The carrier offset is determined by the following equation.${\Delta\quad f} = \frac{\int_{0}^{0^{{+ 2}{MT}}}{\left( {{{phsSingleDiff}(t)} - {\pi/4}} \right){\mathbb{d}t}}}{4\pi\quad{MT}^{2}}$Offset estimation is improved by using burst detection performed by theburst detector module 106. An improved carrier offset is given by thefollowing equation.${\Delta\quad f} = \frac{\sum\limits_{m = 0}^{m = {{2M} - 1}}\left( {{{phsSingleDiff}\left( {k + {mT}} \right)} - {\pi/4}} \right)}{4\pi\quad M\quad T}$The improved carrier offset increases probability of selecting bestsamples for carrier recovery. The carrier recovery module 117 uses bestsamples to recover the carrier signal.

The carrier recovery module 117 decreases a frequency step size of theAFC using the following AFC algorithm. The AFC algorithm utilizes acharacteristic of DQPSK. A relationship between DQPSK and π/4-DQPSK isshown in the following table. Phase difference Transmitted π/4 − DQPSKDQPSK signal Δθ(k) Δθ(k) − π/4 (0, 0)  π/4 0 (0, 1) 3π/4  π/2 (1, 1)−3π/4  π (1, 0) −π/4 −π/2

A DQPSK signal can be obtained by subtracting π/4 from π/4-DQPSK signal.Therefore, the input signal is rotated by π/4 by the phase rotatormodule 117-10. This is mathematically expressed as follows.$\begin{matrix}{\theta_{Pk} = {\theta_{Rk} - {k \cdot {\pi/4}}}} \\{= {\theta_{Tk} + \phi_{ek} + n_{ek} - {k \cdot {\pi/4}}}} \\{= {{I_{k} \cdot {\pi/2}} + \phi_{ek} + n_{ek}}}\end{matrix}$where I_(k)=I_(k−1)+I_(T), φ_(ek) is phase change caused by frequencyoffset, and n_(ek) is phase change caused by Gaussian noise.

The carrier recovery module 117 initializes the AFC with a frequencyoffset within a small predetermined range. Thereafter, the carrierrecovery module 117 decreases the frequency offset based on the improvedcarrier offset provided by the offset estimator module 114. When addtimes and subtract times of the up-down counter module 117-9 are nearlyequal, the PLL is locked, and the carrier filter module 117-5 beginscarrier recovery.

An equation for the carrier filter module 117-5 is expressed as follows.${\overset{\Cap}{\phi}}_{ek} = {\frac{1 - \alpha}{1 - {\alpha\quad z^{- 1}}}\phi_{ek}^{\prime}}$where φ′_(ek) is input phase error, {circumflex over (φ)}_(ek) isfiltered phase error, α=exp(−2/Q), andQ_(k)=10·k/N_(ƒ)(k<N_(ƒ))=10(k>=N_(ƒ)).

The carrier recovery module 117 comprises the phase-shift calculatormodule 117-3 that uses a feedback mechanism, which may be a form ofreverse modulation. A code detected by the detector module 117-4 is fedback to the phase-shift calculator module 117-3. This eliminates a phasechange caused by the phase information in the signal. This ismathematically expressed as follows.φ′_(ek)=(I _(k) −Î _(k))·π/2+φ_(ek) +n _(ek)Î can be obtained by the following equation.Î=[(θ _(Pk)−{circumflex over (φ)}_(ek−1))/(π/2)+½]Thereafter, the differential decoder module 120 decodes the output ofthe carrier recovery module 117. Thus, Î_(T)=Î_(k)−Î_(k−1).

Referring now to FIG. 7, bit timing estimated using correlation,parabolic curve fitting, and interpolation may be accurate at least 65%of the time as compared to known accurate timing. This is shown in thepi-chart by the region having timing offset=0.

Referring now to FIG. 8, a demodulation method 200 used in a PHSreceiver begins at step 202. A downsample module 70 downsamples a signalreceived by the PHS receiver at a rate of three samples per symbol or3f_(b) in step 204. Whether a channel is a control channel or a trafficchannel is determined in step 205.

If the channel is control channel, a burst detector module 106 detects aburst in step 206. The burst detector module 106 determines if the burstis detected in step 208 based on whether moving average of one of threephases (phase 1, phase 2, or phase 3, wherein phase i is an array ofi-th of three samples for every symbol) is less than a predeterminedburst threshold ThB. If true, the burst detector module 106 generates aburst-detect signal in step 210. Otherwise, the burst detector module106 continues to detect burst in step 206.

The burst-detect signal enables a timing module 108 and an offsetestimator module 114 in step 212. The timing module 108 determinescorrect sampling time for the control channel in step 213 based on PR inthe control channel. A sampling module 112 samples symbols in subsequentsamples in step 224 using the sampling time generated by the timingmodule 108.

The offset estimator module 114 estimates a carrier offset in step 226.A carrier recovery module 117 initializes an AFC with an estimated valueof carrier offset in step 228. The AFC checks in step 230 if a PLL islocked. If false, the carrier recovery module 117 decreases a frequencystep size of the AFC in step 232. If true, the carrier recovery module117 locks the AFC in step 234. The demodulation method 200 determines instep 236 that the sample is on time, that is, the sample is the bestsample that comprises a correct symbol value. The demodulation system75-1 demodulates the sample, and a differential decoder module 120decodes a symbol from the sample in step 238. The method 200 restarts instep 202.

On the other hand, if the channel is traffic channel, a UW correlatormodule 110 correlates PR and UW phases in step 214 and generates acorrelation curve in step 216 using correlation samples. To correlateUW, the correlator module 110 uses the burst detection and bit timinginformation of the control channel to estimate UW position in trafficchannels.

The UW correlator module 110 fits the correlation curve to a parabola instep 218. The UW correlator module 110 calculates a peak of the parabolaand a peak time based on an x-coordinate of the peak in step 220. Thepeak time represents the best time to sample a symbol. The UW correlatormodule 110 calculates a timing offset in step 222 based on a distancebetween the peak of the parabola and an adjacent point on thecorrelation curve.

The timing module 108 determines correct sampling time for the trafficchannel in step 223 based on the timing offset. The sampling module 112samples symbols in subsequent samples in step 224 using the samplingtime generated by the timing module 108.

The offset estimator module 114 estimates the carrier offset for thecontrol channel in step 226. To recover carrier in traffic channel, thecarrier recovery module 117 initializes AFC with the estimated value ofthe carrier offset in step 228. The AFC checks in step 230 if the PLL islocked based on the carrier offset. If false, the carrier recoverymodule 117 decreases the frequency step size of the AFC in step 232. Iftrue, the carrier recovery module 117 locks the AFC in step 234 andupdates the carrier offset.

The demodulation method 200 determines in step 236 that the sample is ontime, that is, the sample is the best sample that comprises a correctsymbol value. The demodulation system 75-1 demodulates the sample, and adifferential decoder module 120 decodes a symbol from the sample in step238. The method 200 restarts in step 202.

As can be appreciated, the systems and methods disclosed herein may beused to determine bit timing in any communication system that utilizesTDMA. For example, the systems and methods disclosed herein may be usedto determine bit timing in wireless communication systems, opticalcommunication systems, etc.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, the specification and the following claims.

1. A wireless communication system, comprising: a sampling module thatsamples a first portion and a second portion of a channel using asampling time; a correlator module that selectively correlates saidfirst portion and said second portion, that generates correlationsamples, and that calculates an offset based on said correlationsamples; a timing module that selectively adjusts said sampling timebased on said offset.
 2. The wireless communication system of claim 1wherein said first portion is a preamble (PR) and said second portion isa unique word (UW).
 3. The wireless communication system of claim 1wherein said correlator module selectively correlates said first portionand said second portion when said channel is a traffic channel.
 4. Thewireless communication system of claim 1 wherein said timing moduleadjusts said sampling time based on said offset when said channel is atraffic channel.
 5. The wireless communication system of claim 1 whereinsaid timing module adjusts said sampling time based on said firstportion when said channel is a control channel.
 6. The wirelesscommunication system of claim 1 wherein said correlation modulegenerates a correlation waveform using said correlation samples,determines a peak of said correlation waveform, and calculates saidoffset based on a distance of said peak from one of said correlationsamples.
 7. The wireless communication system of claim 1 wherein saidcorrelation module generates a correlation waveform using saidcorrelation samples and determines a peak of said correlation waveformusing parabolic curve fitting, and calculates said offset based on adistance of said peak from one of said correlation samples.
 8. Thewireless communication system of claim 1 wherein said correlator modulecorrelates said first portion and said second portion for each time slotof said channel.
 9. The wireless communication system of claim 1 whereinsaid correlator module correlates using training symbols in saidchannel.
 10. The wireless communication system of claim 1 wherein saidchannel comprises a plurality of time slots and wherein said timingmodule calculates said offset for each of said time slots.
 11. Thewireless communication system of claim 1 wherein said correlator moduleestimates a location of said second portion in a traffic channel basedon said sampling time of a control channel and based on a burst detectedin said control channel.
 12. The wireless communication system of claim1 further comprising a burst detector module that communicates with saidsampling module, that detects a burst in a control channel, and thatgenerates a burst-detect signal when a moving average of one of N phasesis less than a predetermined threshold, wherein said one of N phases isan array of Nth sample of every symbol in a signal sampled at a samplingrate of N samples per symbol, and N is an integer greater than
 1. 13.The wireless communication system of claim 12 wherein said burst-detectsignal activates said timing module.
 14. The wireless communicationsystem of claim 12 further comprising a carrier offset estimator modulethat communicates with said burst detector module and that generates acarrier offset for a control channel when activated by said burst-detectsignal.
 15. The wireless communication system of claim 14 furthercomprising a carrier recovery module that communicates with saidsampling module and that uses a phase-locked loop (PLL) to recover acarrier signal from a sample output by said sampling module.
 16. Thewireless communication system of claim 15 wherein said PLL isinitialized with said carrier offset and wherein said carrier offset isupdate when said PLL locks for each time slot in said channel.
 17. Thewireless communication system of claim 15 further comprising adifferential decoder module that communicates with said carrier recoverymodule and that decodes a symbol from said sample.
 18. The wirelesscommunication system of claim 1 wherein said sampling module uses one ofa cubic interpolator and a parabolic interpolator.
 19. The wirelesscommunication system of claim 1 wherein an output sample rate of saidsampling module is equal to a symbol rate and wherein an input samplerate of said sampling module is an integer multiple of said symbol rate.20. A personal handy-phone system (PHS) receiver comprising the wirelesscommunication system of claim
 1. 21. A method for operating wirelesscommunication system, comprising: sampling a first portion and a secondportion of a channel using a sampling time; selectively correlating saidfirst portion and said second portion; generating correlation samples;calculating an offset based on said correlation samples; and selectivelyadjusting said sampling time based on said offset.
 22. The method ofclaim 21 wherein said first portion is a preamble (PR) and said secondportion is a unique word (UW).
 23. The method of claim 21 furthercomprising selectively correlating said first portion and said secondportion when said channel is a traffic channel.
 24. The method of claim21 further comprising adjusting said sampling time based on said offsetwhen said channel is a traffic channel.
 25. The method of claim 21further comprising adjusting said sampling time based on said firstportion when said channel is a control channel.
 26. The method of claim21 further comprising: generating a correlation waveform using saidcorrelation samples; determining a peak of said correlation waveform;and calculating said offset based on a distance of said peak from one ofsaid correlation samples.
 27. The method of claim 21 further comprising:generating a correlation waveform using said correlation samples;determining a peak of said correlation waveform using parabolic curvefitting, and calculating said offset based on a distance of said peakfrom one of said correlation samples.
 28. The method of claim 21 furthercomprising correlating said first portion and said second portion foreach time slot of said channel.
 29. The method of claim 21 furthercomprising correlating using training symbols in said channel.
 30. Themethod of claim 21 wherein said channel comprises a plurality of timeslots and further comprising calculating said offset for each of saidtime slots.
 31. The method of claim 21 further comprising estimating alocation of said second portion in a traffic channel based on saidsampling time of a control channel and based on a burst detected in saidcontrol channel.
 32. The method of claim 21 further comprising:detecting a burst in a control channel; and generating a burst-detectsignal when a moving average of one of N phases is less than apredetermined threshold, wherein said one of N phases is an array of Nthsample of every symbol in a signal sampled at a sampling rate of Nsamples per symbol, and N is an integer greater than
 1. 33. The methodof claim 32 further comprising adjusting said sampling time when saidburst-detect signal occurs.
 34. The method of claim 32 furthercomprising generating a carrier offset for a control channel when saidburst-detect signal occurs.
 35. The method of claim 34 furthercomprising using a phase-locked loop to recover a carrier signal from asample output.
 36. The method of claim 35 further comprising:initializing said PLL with said carrier offset; and updating saidcarrier offset when said PLL locks for each time slot in said channel.37. The method of claim 35 further comprising decoding a symbol fromsaid sample.
 38. The method of claim 21 further comprising using one ofa cubic interpolator and a parabolic interpolator.
 39. The method ofclaim 21 further comprising: setting an output sample rate equal to asymbol rate; and setting an input sample rate equal to an integermultiple of said symbol rate.